Calibration theory suggests grayscale gamma is a primary consideration in trying to make various displays look similar using measurements. One complication when relating theory to actual practice are the numerous types of displays, which each have a number of display settings. The various available displays and settings often tend to provide different gamma measurements, depending on the selected measurement pattern. The main question I’ve had for years - does a gamma measurement provide information that can be compared with other displays or settings? Anyone is welcome to follow along and correct where my impressions may fail to fit with available data.
The general forum consensus seems to suggest consistent gamma measurements require different measurement patterns, depending on the display or settings. The main exception are the displays that can return similar measurements regardless of the pattern, like fixed-backlight or fixed-iris models. If you happen to have one of the displays that provide consistent gamma measurements then apparently your information is relevant, but rarely do I read about people making side-by-side visual checks to see if similar measurements on other displays actually compare well against displays with predictable gamma measurements. Essentially the general forum advice seems to be that calibrators need to assemble a compendium of the differing opinion regarding which measurement pattern should be used with each type of display and learn how to apply the information. Personally I cannot remember seeing measurement data to support the suggestion that the easiest way to arrive at similar gamma measurement information is by using dissimilar measurement patterns, so at best I consider the general opinion regarding gamma measurements as difficult to implement for a typical person using an unfamiliar display. Generally I expect that gamma measurements probably often result in limited correlation among various displays, which just means similar gamma measurements may actually result in dissimilar display performance.
Based on the measurements I have seen, it appears that various displays and settings primarily operate differently depending on how information is presented to the display, and I do not see a reason that unique measurement patterns are necessarily required for generally comparable gamma measurements on a majority of displays. Certainly if you only want to take a single gamma measurement run it makes sense to carefully consider how to assemble a gamma measurement pattern to meet your intentions, but I see no clear reason to doubt if holding a constant average picture level (APL) and a constant average luminance (AL) during a measurement run provides a single point of comparable gamma information across various displays and settings. Personally I have not measured patterns that only intend to hold a constant average luminance (AL), yet based on the preliminary notes for the upcoming Spears and Munsil disc
, it would appear that everyone is not tied to the idea that various displays require dissimilar measurement patterns for gamma.
By this point I don’t know if anyone is still following along, but I’ll explain how I view some of the plasma gamma measurement data posted in the last few months. Since I don’t have access to the plasma displays with an opportunity to make subjective comparisons, like viewing the plasma display side by side with an LCD that has a fixed-backlight, I choose to limit which data I consider most relevant. In my own measurements on non-plasma displays, gamma measurements that hold APL & AL constant seem to have the best correlation between gamma measurement data and my subjective impression when compared to how an LCD-based display with a fixed-backlight would measure. Some displays can clearly vary relative gray measurements as APL or AL changes, so if APL & AL are constant during the measurement run there are simply fewer variables to possibly influence the reported gamma information on displays that do not provide consistent measurements regardless of pattern.
Primarily I’m just going to discuss the plasma measurements from Chad B (See linked thread).
The items he measured that hold a constant APL & AL during the measurement run are listed below from darkest to brightest (See linked PDF for graphs)
- DB Ramp Dark (Darkest image)(4% APL & 3% AL)
- AVS S APL (Dark image)(7% APL & 5% AL)
- Chad B’s APL (Typical image)(19% APL & 13% AL)
- AVS L APL (Bright image)(36% APL & 25% AL)
- DB Ramp Bright (Extremely bright image)(85% APL & 84% AL)
When I look at the listed gamma graphs I compare them to each other for general shape and how they are located vertically in relation to the horizontal reference line. Generally most of the listed gamma graphs approximate to a rising line between 20% and 60% gray, and the graphs are generally above the colored horizontal reference line. There are a couple anomalies on the brighter graphs, where the AVS L APL has a dip at 80% gray and the DB Ramp Bright has a greater dip at 90% gray. I don’t know if I would be able to easily notice possible changes on brighter scenes with typical video content, but I suspect that I would be able to notice how the display changes gamma between dark and bright scenes in a dim room, which is the increase of approximately 0.2 gamma for 20%-60% gray when comparing the DB Ramp Dark graph to the AVS L APL graph.
Based on the measurements from Chad B, I would assume that when his plasma is calibrated using the 21% APL patterns (closest to reference) it causes the on-screen image (5 patterns listed above) to generally have a higher gamma than the 21% APL measurement indicates. Chad B’s APL and AVS L APL measurements are the most above reference, so I would expect typical scenes and bright scenes to have the most pronounced difference compared to an LCD calibrated with the same 21% APL pattern. Personally I doubt if using the 21% APL pattern to calibrate both the given plasma and an LCD with a fixed-backlight would tend to make identical gamma measurements result in similarly-looking images if the two displays were viewed side-by-side, since the on-screen measurements (5 patterns listed above) suggest that the plasma might tend to actually have a higher gamma.
If the five listed graphs from Chad B were each averaged to a single value, they would appear to fit with how Zoyd has documented the way his plasma changes average gamma depending on the brightness of the image displayed (See link)
. The measurements from Chad B and the information from Zoyd seem to suggest that on-screen gamma changes on their plasmas to some extent depending on the image displayed. The information from Zoyd also suggests that the variation on his plasma is somewhat independent from how the display was calibrated, since the graphs for both the 15% windows and his 1% area patterns generally parallel each other. I have read suggestions on this forum that historically windows have been used for gamma measurements because they allow a user to calibrate for a wide range of image brightness, but the information from Zoyd or Chad B doesn’t appear to support such suggestions, since for example the on-screen measurements from Chad B do not appear to generally relate with the window measurements. Unfortunately I don’t believe current consumer-level software can easily provide the amount of display-specific data provided in Zoyd’s graph, and in order to similarly document display performance would likely require a lot of time or automation of some sort.
I cannot come to the sort of one or two sentence conclusion that the average forum reader will likely want, regarding how they should attempt to measure and calibrate gamma. My own interest was mainly to see if anyone has additional factual data that might suggest how well or poorly gamma measurements may coincide. I am aware of deficiencies unrelated to gamma for displays that I have tried measuring closely, so maybe it should not be shocking to have measurements suggesting that modern plasma might vary on-screen gamma by a 0.1-0.2 range somewhat regardless of how the display is measured and calibration attempted. For the people that invariably want to know what single measurement series they should use to calibrate their display, I still get the impression that multiple measurement runs can give more compete information about display performance. I consider it unfortunate that measurement software does not appear headed in that direction, so I'll hope that the idea behind the measurement patterns on the future Spears and Munsil release provides better correlation for gamma measurements across various displays and settings than my impression of the current situation.